It is increasingly recognised that integrated optical circuits have a number of advantages over electrical circuits. However, it has been difficult to produce integrated optical circuits which are comparably small, primarily due to the difficulty in producing waveguides which can include tight bends without large signal losses. It has also been difficult to produce integrated optical circuits including signal processing devices which can be easily coupled to current optical fibres, owing to a difference in the refractive index of the material used for optical fibres and those materials typically used for integrated optical devices, whilst still maintaining compact sizes.
Photonic crystals typically comprise a lattice of air holes formed in a core material, typically silicon, and exhibit a photonic bandgap. By not including some holes in the lattice, a defect state waveguide can be formed. Confinement of light within the waveguide is provided by using light within the photonic bandgap wavelength range. However, it has been found that devices of this type suffer from large losses, mainly due to the escape of light from the waveguide in a vertical direction. Furthermore, in order to provide a strong and complete bandgap at optical frequencies, it has been necessary to use a high refractive index material, typically silicon. This makes it difficult to couple light into the waveguides from existing optical fibres, which typically have a core having a much lower refractive index. This problem necessitates complicated, lossy mode coupling devices.
The object of the present invention is to reduce losses from photonic crystals and to allow easy coupling of photonic crystal structures to existing waveguides.